feat(artist): "new since last visit" badge + banner

Per-artist "+N" accent pill on the artists directory and a "N new since
last visit" banner inside ArtistView. Counts new IMAGES (not posts) so
multi-image posts increment correctly.

- alembic 0034: artist_visit (artist_id PK, last_viewed_at NOT NULL).
  Seeds every existing artist with last_viewed_at=NOW() so the badge
  starts at 0 across the board — no noisy "5000 unseen images" on
  first deploy.
- ArtistService.find_or_create autoseeds a visit row alongside new
  artists, so freshly imported content doesn't read as unseen.
- ArtistService.overview reads pre-visit last_viewed_at, counts images
  created since, then atomically UPSERTs last_viewed_at=NOW() via
  postgres ON CONFLICT DO UPDATE (no SELECT-then-INSERT race per
  reference_scalar_one_or_none_duplicates). Returns the pre-update
  count as `unseen_count_at_visit` so the banner has data.
- ArtistDirectoryService.list_artists adds an `unseen_count` aggregate
  to each card via LEFT JOIN artist_visit + conditional COUNT. NULL
  last_viewed_at (artist created before this code shipped) defensively
  counts as "never visited" → all images unseen.
- Frontend: ArtistCard renders an accent pill in the preview-strip
  corner when unseen_count > 0 (capped at 99+); ArtistView shows a
  closable v-alert banner on initial load when
  unseen_count_at_visit > 0, re-arms on slug change.

Single-row-per-artist (no user_id) — rule #47 multi-user ACL is
aspirational; widens to (user_id, artist_id) PK when User lands, per
rule #22.

Scribe plan #597.

backend/app/services/artist_directory_service.py

@@ -13,10 +13,10 @@ from __future__ import annotations
 import base64
 from dataclasses import dataclass
 
-from sqlalchemy import and_, exists, func, or_, select
+from sqlalchemy import and_, case, exists, func, or_, select
 from sqlalchemy.ext.asyncio import AsyncSession
 
-from ..models import Artist, ImageRecord, Source
+from ..models import Artist, ArtistVisit, ImageRecord, Source
 from .gallery_service import thumbnail_url
 
 _SEP = "|"
@@ -58,9 +58,27 @@ class ArtistDirectoryService:
             raise ValueError("limit must be between 1 and 200")
 
         count_col = func.count(ImageRecord.id).label("image_count")
+        # Unseen = images imported since the artist's last_viewed_at.
+        # NULL last_viewed_at (artist created before alembic 0034 seed
+        # or before find_or_create autoseed) defensively counts as
+        # "never visited" → all images unseen. Single grouped query, no
+        # N+1.
+        unseen_col = func.count(
+            case(
+                (
+                    or_(
+                        ArtistVisit.last_viewed_at.is_(None),
+                        ImageRecord.created_at > ArtistVisit.last_viewed_at,
+                    ),
+                    ImageRecord.id,
+                ),
+                else_=None,
+            )
+        ).label("unseen_count")
         stmt = (
-            select(Artist, count_col)
+            select(Artist, count_col, unseen_col)
             .outerjoin(ImageRecord, ImageRecord.artist_id == Artist.id)
+            .outerjoin(ArtistVisit, ArtistVisit.artist_id == Artist.id)
             .group_by(Artist.id)
         )
         if q:
@@ -94,7 +112,7 @@ class ArtistDirectoryService:
             next_cursor = _encode(last_artist.name, last_artist.id)
             rows = rows[:limit]
 
-        artist_ids = [a.id for a, _ in rows]
+        artist_ids = [a.id for a, _, _ in rows]
         previews = await self._previews(artist_ids)
 
         cards = [
@@ -104,9 +122,10 @@ class ArtistDirectoryService:
                 "slug": artist.slug,
                 "is_subscription": bool(artist.is_subscription),
                 "image_count": int(image_count),
+                "unseen_count": int(unseen_count),
                 "preview_thumbnails": previews.get(artist.id, []),
             }
-            for artist, image_count in rows
+            for artist, image_count, unseen_count in rows
         ]
         return DirectoryPage(cards=cards, next_cursor=next_cursor)